Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems.
By way of example, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple user equipments (UEs; e.g., mobile devices). A base station may communicate with UEs on downlink channels (e.g., for transmissions from a base station to a UE) and uplink channels (e.g., for transmissions from a UE to a base station).
Some modes of communication may enable communications with a UE over different radio frequency spectrum bands (e.g., a licensed radio frequency spectrum band and/or an unlicensed radio frequency spectrum band). With increasing data traffic in cellular networks, the offloading of at least some data traffic from a licensed radio frequency spectrum band to an unlicensed radio frequency spectrum band may provide a cellular operator with opportunities for enhanced data transmission capacity.
Prior to gaining access to and transmitting data over the unlicensed radio frequency spectrum band, a transmitting apparatus may, in some examples, perform a listen before talk (LBT) procedure to gain access to the unlicensed radio frequency spectrum band. An LBT procedure may include performing a clear channel assessment (CCA) to determine whether a particular channel of the unlicensed radio frequency spectrum band is available. When it is determined that the channel of the unlicensed radio frequency spectrum band is not available (e.g., because another device is already using the channel of the unlicensed radio frequency spectrum band), a clear channel assessment may be performed for the channel again at a later time.
When a base station performs a clear channel assessment and gains access to an unlicensed radio frequency spectrum band, it may assume that a UE will receive its transmission and respond appropriately. However, situations may arise when a base station does not win contention to access the unlicensed radio frequency spectrum band; when a UE incorrectly determines that a clear channel assessment performed by a base station has failed; or when a UE incorrectly determines that a clear channel assessment performed by a base station was successful. In these and other situations, a UE may respond to a base station in a manner that is unexpected and potentially ambiguous.